The primary focus of our study is to investigate the biochemical and molecular defense mechanisms against carcinogens and xenobiotics in human breast cancer MCF-7 cells. The induction of neoplasm by carcinogens has been well documented in animal model, e.g. benzopyrene (BP) for liver cancer and dimethylbenzanthracene (DMBA) for mammary cancer. We chose to examine the mechanism of action of carcinogen resistance in our series of BP resistant cells. In our BP resistant cells, we found CYP1A1 activation by BP, DMBA, and TCDD was abolished. Topoisomerase II expression was enhanced with resistance. Our results suggested that the signal transduction pathway for carcinogen activation, aryl hydrocarbon receptor ( AhR) may be directly related to the carcinogen resistance. Whether AhR is also involved in BP resistant cells is under our current investigation. The detoxification mechanism in our BP resistant human breast cancer MCF-7 cells and the regulation by dietary flavonoids are other areas to be focused.